Surgical reconstruction of the anterior cruciate ligament (ACL) has become the treatment of choice for many ACL-deficient individuals wishing to maintain an active lifestyle. This procedure, though beneficial to many, is plagued by inconsistent outcome and can fail to protect against long-term joint degeneration. The key to successful treatment of ACL injury is restoration of dynamic knee stability; i.e. stability during functional movement. Implantation of a replacement graft does not guarantee success. The mechanical properties of ACL grafts deteriorate after implantation due to biological remodeling. Neuromuscular control of the ACL-reconstructed knee is impaired due to loss of sensory input from the intact ACL. The effects of these factors on graft and knee function are unknown. In fact, little is known about the dynamic behavior of the ACL-reconstructed knee, since traditional motion analysis techniques are incapable of reliably measuring knee stability during functional activities. This has hampered efforts to design optimal rehabilitation strategies, identify mechanisms leading to graft failure and improve outcome. This study will employ novel technology to study function of the ACL-injured and contralateral (uninjured)knees of subjects after ACL reconstruction surgery. A high frame-rate (250 frame/s), accurate (plus or minus 0.1 mm) biplane digital radiography system will be used along with dynamic electromyography to assess three-dimensional knee motion and neuromuscular function during movements known to stress the ACL (single-leg hopping, downhill running) at 4, 8 and 24 months after reconstruction surgery. Standard measures of clinical function (IKDC, VAS and Cincinnati Knee Ligament Rating System) will also be acquired. Accelerated and conservative rehabilitation strategies will be compared. This will enable evaluation of the effectiveness of ACL reconstruction surgery for restoring dynamic stability, the time course of changes in dynamic knee stability and control following ACL reconstruction, the nature and importance of compensatory neuromuscular strategies, and the effects (if any) of accelerated rehabilitation on dynamic knee stability. Findings from this study will facilitate the design of optimal postoperative rehabilitation strategies and the development of approaches to improve outcome following surgical ACL repair.